It has long been known in the packaging industry to employ packaging materials comprising core layers of, for example, paper or paperboard and outer layers of thermoplastic, which outer layers may be sealed to one another by fusion after heating and compression, so as obtain tight and durable sealing joints or seams.
According to one method of production of single-use disposable packages which has been put to large scale use, a web of packaging material is formed into a tube in that the longitudinal edges of the web are united to one another in an overlap joint and the web edges are sealed to one another. The thus formed tube can then be filled with the intended contents, such as, for example, liquid or pumpable foods, whereafter the filled tube is divided into individual packaging containers by transverse sealing of the tube along narrow zones which are applied in uniform spaced apart relationship from one another. Thereafter, the closed tube sections are separated from the rest of the tube, for the formation of individual packaging containers.
The sealing of the packaging material will be considerably facilitated if the packaging material includes a layer of metal foil, preferably aluminium foil, which also constitutes the barrier layer of the packaging material and protects the enclosed product. By inducing, in the aluminium layer of the packaging material, electric currents with the aid of a so-called inductor or coil which basically consists of an electric conductor formed into the desired configuration and forming one or more coil turns which are connected to a current source which supplies the coil with high frequency alternating current. Frequencies of 500 kHz and 1.8 MHz are generally employed.
When a high frequency current is passed through the coil or inductor, a high frequency magnetic field occurs around the coil or inductor and, if the coil is placed adjacent a material which includes a metal layer, induction currents are induced in this metal layer which give rise to a heat generation in those parts where the current is led. The heat generated in the metal foil layer is readily conducted over to adjacent layers of thermoplastic such as, for example, polyethylene, which are thereby caused to soften or melt. If the material layers in which heat is generated are compressed against another similar material layer with a thermoplastic coating, the thermoplastic coatings will, in the region of the heating, fuse together for the formation of a tight and durable seal joint.
In order to concentrate the magnetic field and obtain narrow heating zones, it is appropriate to dispose the inductor as close to the metal layer as possible, i.e. the inductor should be pressed against the material which is to be sealed. In order further to improve the sealing joint, the inductor may be designed so that the surface of the inductor facing towards the packaging material is provided, along a part of its surface, with a projecting ridge in order further to amplify the sealing pressure. An inductor according to the above description is disclosed in, for example, Swedish Patent Specification SE-451 974.
One problem inherent in all inductors of the above-outlined type is, however, the difficulty in obtaining a uniform seal transversely across the entire surface which is constituted by the flat-laid packaging material tube so that both the central zone of the transverse seal and the edge zone will have a tight and durable seal. When the induced current reaches the edge of the packaging material web, there is nothing to conduct the current further, with the result that, when the current approaches the packaging material edge, it deflects gradually off to the opposite side where the current is counter-directed. There will thus be a lower current density and thereby lower power output density furthest out towards the edge of the packaging material. Previously, this has been compensated for by increasing the power output so that it is sufficient also to seal right out to the outermost edge of the packaging material.
For packed products of a liquid nature, such as, for instance water, juice or milk, a power output increase to compensate for reduced power output density at the edge of the material web functions quite satisfactorily, but for thick or viscous products such as, for example, tomato puree or paste and the like, it has proved extremely difficult to obtain a tight and uniform seal joint transversely across the entire packaging material tube, since it is not possible to raise the power output limitlessly. An excessive high power output will negatively affect the central region of the packaging material tube, in that the thermoplastic is converted in plastic droplets, so-called plastic spray, which may affect the tightness and quality of the finished package.